1. Field of the Invention
The present invention relates to a mechanism for correcting unbalance of a rotor that occurs when leading and trailing edges of a sheet member are held on a peripheral surface of the rotor and the rotor is rotated.
2. Description of the Related Art
Generally, a photosensitive printing plate (hereinafter, xe2x80x9cprinting platexe2x80x9d), comprising a sheet-like support (e.g., a thin aluminum plate) having disposed thereon a photosensitive layer, is used for printing. Printing plates having different vertical and horizontal dimensions are used depending upon the size of the item to be printed.
An image exposure apparatus is used to image-expose the printing plate. In one such image exposure apparatus, the printing plate is wound around a rotating drum and irradiated with a light beam corresponding to image data while the printing plate is rotated integrally with the rotating drum, whereby the printing plate is scan-exposed.
When the printing plate is wound around the rotating drum, ends of the printing plate along the circumferential direction of the rotating drum are nipped by a chuck and fixed between the chuck and the periphery of the rotating drum.
Specifically, a holding device (i.e., a chuck) corresponding to one end (e.g., a leading edge in the direction in which the printing plate is wound) of the printing plate along the circumferential direction of the rotating drum is attached at a predetermined position on the rotating drum. After the printing plate is wound around the rotating drum, a holding device (i.e., a chuck) corresponding to the other end (i.e., a trailing edge) of the printing plate is attached at a position on the rotating drum according to the size of the printing plate.
Since the chucks are attached at positions on the periphery of the rotating drum that differ depending on the size of the printing plate wound on the rotating drum, the balance of the rotating drum varies. For this reason, eccentricity occurs when the rotating drum rotates at high speed, which can cause deterioration in the quality of the image recorded on the printing plate.
In order to solve this problem, there has been the proposal to use an anchor and adjust the position at which the anchor is attached to the rotating drum in accordance with the size of the printing plate, to thereby adjust the balance of the rotating drum. Balance of the rotating drum is adjusted by multiplying the weight of the anchor and the distance from a rotary shaft to the position at which the anchor is attached (weightxc3x97distance=moment) to determine moment, and altering the moment (by altering at least one of the anchor weight and the distance) in accordance with the size of the printing plate.
However, with conventional structures, the position of the anchor must be adjusted each time the size of the printing plate varies, and the adjustment takes time and adversely affects processing efficiency. Moreover, the mechanism by which the position of the anchor is adjusted is complicated and the number of parts increases, whereby the weight of the rotating drum itself increases, and higher output power becomes necessary to rotate the drum at high speed, whereby costs escalate.
In consideration of the aforementioned facts, an object of the present invention is to provide a mechanism that is capable of correcting unbalance of a rotor by disposing, at a predetermined distance from a rotary shaft, an anchor having a fixed weight to correct unbalance caused by positions at which at least one of a leading edge chuck and a trailing edge chuck that hold a sheet member disposed on a peripheral surface of the rotor varying in accordance with the size of the sheet member.
To achieve the object mentioned above, according to one aspect of the present invention, there is provided a rotor, disposed with an unbalance correcting mechanism and around which a sheet member is wound and fixed, comprising: (a) a rotor body, including an axis of rotation and a peripheral surface for supporting the sheet member; (b) a chuck device for pressing a leading edge and a trailing edge of the sheet member against the peripheral surface of the rotor body, the chuck device including a first chuck and a second chuck and having a first mode, in which the first chuck is attached to the rotor body and the second chuck is apart from the rotor body, and a second mode, in which both the first and second chucks are attached to the rotor body; (c) a main balancer attached to the rotor body and having a first relative positional relation with the first chuck; and (d) a sub-balancer attached to the rotor body and having a second relative positional relation with the second chuck in the second mode; wherein the main balancer and the sub-balancer increase unbalance of the rotor in the first mode and reduce unbalance of the rotor in the second mode.
In accordance with another aspect of the present invention, there is provided an apparatus for forming an image on a printing plate, comprising: (I) a drum around which a printing plate is taken up and fixed, the drum including (a) a drum body, including an axis of rotation and a peripheral surface for supporting the sheet member, (b) a chuck device for pressing a leading edge and a trailing edge of the sheet member against the peripheral surface of the drum body, the chuck device including a first chuck and a second chuck and having a first mode, in which the first chuck is attached to the drum body and the second chuck is apart from the drum body, and a second mode, in which both the first and second chucks are attached to the drum body, (c) a main balancer attached to the drum body and having a first relative positional relation with the first chuck, and (d) a sub-balancer attached to the drum body and having a second relative positional relation with the second chuck in the second mode, wherein the main balancer and the sub-balancer increase unbalance of the rotor in the first mode and reduce unbalance of the rotor in the second mode; (II) a section for feeding the printing plate to the drum; (III) a section for rotating the drum; (IV) a section for recording an image onto the printing plate wound around the periphery of the drum body; and (V) a section for detaching the printing plate from the drum.